1. Field of the Invention
The present invention relates to an image forming device such as a copier, printer, or fax machine, and in further detail relates to an image forming device which can easily eliminate static electricity and separate a sheet like recording medium, onto which a toner image has been transferred, from an image supporter.
2. Background Information
Generally, in image forming devices such as electrophotographic copiers, printers, and fax machines, a toner image is formed or transferred to an image supporter such as a photosensitive drum or an intermediate transfer medium, and then the toner image which has been formed or transferred to the surface of the image supporter is transferred to a sheet like recording medium (such as copy paper or a plastic film). Next, this recording medium is separated from the surface of the image supporter by separating means, and then the recording medium which has been separated is sent to fixing means. Furthermore, after the toner image has been fixed on the recording medium by the fixing means, the fixed recording medium is transported to a discharge tray or an intermediate tray for double sided printing.
The discharge separation system shown in FIG. 8 (Refer to Japanese Patent H09-218623 (Patent Reference 1)), the discharge separation system shown in FIG. 9 (Refer to Japanese Patent H08-137358 (Patent Reference 2)), and the discharge separation system shown in FIG. 10 (Refer to Japanese Patent H07-40158 (Patent Reference 3)) are commonly known separating means for this type of image forming device. Of these systems, the separating means 100, 101 shown in FIG. 8 and FIG. 9 have a plurality of separation needles 102, 103 which are supported by an upstream wall region 104, 105 located upstream in the recording medium transport direction, and by a downstream wall region 106, 107 located downstream in the recording medium transport direction. Furthermore, the plurality of separation needles 102, 103 are positioned in a direction orthogonal to the recording medium transport direction (Hereinafter referred to as the paper transport lateral direction). Furthermore, the separating means 108 shown in FIG. 10 has a separation needle 110 with a plurality of discharge tips 110a at prescribed intervals along the longitudinal direction of the image supporter 114 located downstream in the sheet transport direction of the transfer charger 109 which is located across from the image supporter 114, and when an electrical charge is applied to the separation needle 110, a discharging current from the separation needle 110 will flow so that the recording medium 112 will separate from the image supporter 114.
With the separating means 100 disclosed in Patent Reference 1, a separating voltage is applied to the separation needle 102 at prescribed timing after the toner image has been transferred to the recording medium 111. Therefore, corona discharge will occur between the separation needle 102 and the image supporter 113, and the static electricity charged on to the recording medium 111 during the copying operation can be discharged by a transfer roller 117. Therefore, the recording medium 111 will separate from the image supporter 113 because of the stiffness and weight of the recording medium.
Furthermore, with the separating means 101 disclosed in Patent Reference 2, excess static charge is discharged from the back side of the recording medium 115 to a grounded separation needle 103, and the static electricity charged to the recording medium by the transfer roller 120 during the transfer operation will be eliminated. Even if this separation means 101 is used, static elimination and separation of the recording medium and the image supporter will be possible if the recording medium is stiff.
Furthermore, with the separating means 108 disclosed in Patent Reference 3, the recording medium 112 will separate from the image supporter 114 after the toner image on the image supporter 114 has been transferred to the recording medium 112. As a condition for separation at this time, when a prescribed voltage is applied to the separation needle 110 and the separation needle 110 and the image supporter 114 are placed at a prescribed interval, the surrounding impedance will change when a transfer corona is generated by the transfer charger 109, a discharge current from the separation needle 110 will be observed, and the recording medium 112 will be separated from the image supporter 114.
Incidentally, in recent years, printing on both sides of the recording medium has become more common for the purpose of effectively using natural resources or the like. When double sided printing is performed on the recording medium in this manner, the recording medium 115 may not readily separate from the image supporter 116 when using the conventional separating means 101 disclosed in Patent Reference 2. Therefore the following measures are taken when printing on both sides of the recording medium 115. The first measure is to first print on one side of the recording medium 115, then use a two sided printing unit or the like to invert the front and back sides of the recording medium 115 which has been printed on one side, and then continue printing on the unprinted side of the recording medium 115. The other measure is to reuse copy paper or the like which has been printed on one side as the recording medium, and then printing on the unprinted side of the recording medium 115 to be reused.
When double sided printing is performed as described above, the curl of the recording medium 115 such as copy paper will tend to be larger than when the recording medium 115 is printed only on one side. At this time, if the curl of the recording medium 115 is able to readily wrap around the side of the image supporter 116 such as a photosensitive drum, or in other words, if the curl is towards the image supporter 116 side, as the curl increases, separating the recording medium from the image supporter 116 will become that much more difficult, the recording medium will wrap around to the image supporter 116 side, and there is a possibility of problems such as paper jams or the like.
In order to relieve these types of problems, with the separating means 100, 108 of Patent Reference 1 and Patent Reference 3 for example, static elimination of the recording medium 111, 112 by applying a voltage which has reverse polarity to the transfer current to the separation needles 102, 110 and causing corona discharging between the separation needle 102, 110 and the image supporter 113, 114 is effective for increasing the efficiency of separating the recording medium 111, 112 from the image supporter 113, 114.
However, with the separating means 100 of Patent Reference 1, if the upstream side wall region 104 and the downstream side wall region 106 are formed from an insulating plastic, the upstream side wall region 104 and the downstream side wall region 106 which are made from insulating plastic will become charged by the corona discharge generated between the separation needle 102 and the image supporter 113, and the electric field (electric field in the paper transport lateral direction) around the plurality of separation needles 102 may be inconsistent depending on the location in the paper transport lateral direction. At this time, if the voltage applied to the separation needle 102 is increased in order to obtain the desired separation properties of the recording medium 111, an abnormal discharge may occur between the separation needle 102 and the image supporter 113, and there is a possibility that dielectric breakdown will occur in the image supporter 113 such as a photosensitive drum.
Similarly, with the separating means 108 of Patent Reference 3, the gap or the like between the separation needle 110 and the image supporter 114 is set to a prescribed value in order to discharge current, and therefore the gap between the separation needle 110 and the image supporter 114 must be precisely set because the separating conditions will vary depending on the value of the voltage applied to the transfer charger 109 and the separation needle 110. Furthermore, if the separation voltage is increased in order to prevent poor separation of the recording medium, there is a possibility that dielectric breakdown to the photosensitive drum will occur if an amorphous silicon (A-Si) photosensitive drum is used, thus causing image defects.
Therefore, the present invention provides an image forming device which can reduce the discharge voltage by increasing the discharge efficiency of the separating means, make the discharge of the separating means uniform in the paper transport lateral direction, prevent dielectric breakdown of the image supporter, and enable good separation of the recording medium from the image supporter.